1. Field of the Invention
The present invention relates to an optical filter for selecting a wavelength to be used in an optical communication and an optical switching equipment and, more particularly, to an optical filter preventing a sidelobe deteriorating its characteristics, using pair grating inserted between waveguides.
2. Description of Related Art
Generally, the Grating-Assisted Vertical Codirectional Coupler (GACC) filter fabricated on semiconductor materials has been used as an optical filter because the semiconductor material has a good integration with optical elements and has an advantage of a fast wavelength variation.
As shown in FIG. 1, a conventional GACC includes a first waveguide (InGaAsP) 12 formed on an n-InP substrate 11, an n-InP cladding layer 14 formed on the first waveguide 12, a second waveguide (InGaAsP) 15 formed on the n-InP cladding layer 14, a p-InP cladding layer 16 formed on the second waveguide 15 and gratings 13 inserted into the n-InP cladding layer 14.
This GACC optical filter generates an optical coupling by inserting gratings 13 into the n-InP cladding layer 14 which is between the first and second waveguides 12 and 15. Accordingly, the conventional GACC optical filter extracts optical signal incident from a waveguide and then uses the extracted signal in another waveguide as an optical signal. At this time, the gratings 13 serves as a coupler to couple optical signals between the first and second waveguides 12 and 15. When the first and second waveguides 12 and 15 are not symmetrical, only a specific wavelength is coupled to another waveguide.
The conventional GACC optical filter is able to alter the wavelength of light from the second waveguide 15, by changing the refractive index of the waveguide using the current injection through the pn junction or the electric effect. In this wavelength-variable optical filter, if the (i+1)-th or (i-1)-th wavelength is outputted to the second waveguide 15 while the i-th wavelength (.lambda.) is outputted to the second waveguide 15 or vice versa, an error may be generated in detecting the i-th wavelength.
Accordingly, in order to output only the i-th wavelength to the second waveguide 15, the optical wavelength spectrum coupled from the first waveguide 12 to the second waveguide 15 should not be outputted at the (i+1)-th or (i-1)-th wavelength.
However, in the structure of the optical filter, as shown in FIG. 1, the output spectrum coupled to the second waveguide 15 is shown in dotted line of FIG. 4 to the optical wavelength change. The coupling power has small peak values on the left and right sides of the middle wavelength, what is called "sidelobe." Since neighboring channels are affected along the sidelobe, it is essential to remove the sidelobe in implementing an available optical filter. This sidelobe arises from the finite coupling length of the filter device and the constant efficiency of the optical coupling therebetween.